Quantitative immunohistochemical analyses were performed on 22 nerve biopsy specimens from patients with systemic vasculitis (n = 14) or isolated vasculitis of peripheral nerve (n = 8). In the vascular lesions the cellular infiltrates were composed primarily of T cells (71 +/- 18%; mean +/- SD) and macrophages (27 +/- 17%), and the majority of the T cells (65 +/- 20%) were cytotoxic/suppressor CD8 cells. B cells were seen in only 4 cases and constituted less than 2% of all cells. Natural killer cells and polymorphonuclear leukocytes were rare, and a leukocytoclastic response was not observed. Fourteen biopsy specimens had vascular deposits of immunoglobulins G and M and complement components C3 and C5b-9 membrane attack complex, while 4 had only the latter. The fact that the immunoglobulin and complement deposits were seen only in vessels that had corresponding intense cellular infiltrates suggests an important, but perhaps not primary, role for immune complexes in causing the vascular lesions. Statistical analysis revealed striking similarities in the lesions of patients with isolated nerve vasculitis and those with systemic vasculitides, suggesting a common pathogenic mechanism. Collectively, our observations suggest an important role for a T-cell-dependent cell-mediated process as a primary mechanism of vessel injury in peripheral nerve vasculitis.